Information Technology Reference
In-Depth Information
Figure 6.10:
A remultiplexer creates a new transport stream from selected programs in other transport streams.
Remultiplexing is easier when all the incoming programs have the same bit rate. If a suitable combination of
programs is selected it is obvious that the output transport stream will always have sufficient bit rate. Where
statistical multiplexing has been used, there is a possibility that the sum of the bit rates of the selected programs
will exceed the bit rate of the output transport stream. To avoid this, the remultiplexer will have to employ
recompression.
Recompression requires a partial decode of the bitstream to identify the DCT coefficients. These will then be
requantized to reduce the bit rate until it is low enough to fit the output transport stream.
Remultiplexers have to edit the Program Specific Information (PSI) such that the Program Association Table (PAT)
and the Program Map Tables (PMT) correctly reflect the new transport stream content. It may also be necessary to
change the packet identification codes (PIDs) since the incoming transport streams could have inadvertently used
the same values.
When Program Clock Reference (PCR) data are included in an extended packet header, they represent a real-time
clock count and if the associated packet is moved in time the PCR value will be wrong. Remultiplexers have to
recreate a new multiplex from a number of other multiplexes and it is inevitable that this process will result in
packets being placed in different locations in the output transport stream than they had in the input. In this case the
remultiplexer must edit the PCR values so that they reflect the value the clock counter would have had at the
location the packet at which the packet now resides.
